In certain subterranean formations, fluid is injected into the reservoir to displace or sweep the hydrocarbons out of the reservoir. This method of production is generally referred to as a method of “Enhanced Oil Recovery” which may be water-flooding, gas injection, steam injection, etc. For the purpose of this specification, the general process will be defined as injecting a fluid (gas or liquid) into a reservoir in order to displace the existing hydrocarbons into a producing well. The primary issue with injecting fluid to enhance oil recovery is how to sweep the reservoir of the hydrocarbon in the most efficient manner possible. Because of geological differences in a reservoir, the permeability may not be homogenous. Because of such permeability differences between the vertical and horizontal directions or the existence of higher permeability streaks, the injecting fluid may bypass some of the reservoir fluid and create a path into the producing well. Even with homogenous reservoirs, the tendency of the injected fluid is to breakthrough into the producing well and consequently leave a large volume of the reservoir un-swept by the injecting fluid. This problem generally gets worse as the mobility ratio between the fluids becomes unfavorable, such as when the mobility of the injected fluid is significantly higher than the reservoir fluid.
The industry has come up with numerous methods to improve the sweep efficiency and the overall reservoir that is swept by individual wells. These methods include fracturing and the use of horizontal wells. The industry currently uses horizontal wells as injectors in an attempt to expose more of the reservoir to the injecting fluid. The goal is to create a movement of injection fluid evenly across the reservoir. This is done to emulate the highly efficient line drive. The industry also uses horizontal wells as producers, again the goal being to evenly produce the reservoir so to form a line drive.
SPE Paper 84077 presents a method referred to as toe-to-heel waterflooding where a horizontal lateral is used to produce the reservoir with a vertical injector located nearer the toe (end) of the lateral. The method referred to in this paper is limited, since the horizontal lateral only covers a limited area in the reservoir. A horizontal lateral covers a small area in the vertical direction, thus the vertical sweep efficiency is fairly low. It therefore does not maximize the amount of surface area that can be used to recover the hydrocarbons. This method also suffers from an inability to control the influx of injection fluid at the toe to improve recovery.
Part of the efficiency of the sweep is reducing the production of the injection fluid. The industry has created several techniques from the use of chemicals that block the injection fluid, to injection fluids that improve the matrix flow through the reservoir to reduce channeling. Some injection programs include attempts to plug high permeability streaks and natural fractures in the reservoir. This is done to shut-off pathways that can exist between the injector well and producing wells. As these pathways are restricted the injection fluid will develop new pathways to the producing wells. This will force the injection fluid into more of the reservoir to displace hydrocarbons, thus improving sweep efficiency and reducing the influx of injected fluid into the producing wells.
When the injection fluid is produced, such as water, it is usually removed from the hydrocarbons at the surface using multi-phase separation devices. These devices operate to agglomerate and coalesce the hydrocarbons, thereby separating them from the water. A drawback of this approach, however, is that no separation process is perfect. As such, some amount of the hydrocarbons always remains in the water. This can create environmental problems when disposing of the water, especially in off-shore applications. Also, the multi-phase separation devices are rather large in size, which is another disadvantage in off-shore applications, as space is limited. Yet another drawback is that these devices can require additional maintenance or repair if solids are part of the produced fluid stream. A further, and perhaps greatest drawback of these solutions, is that they do nothing to increase or maximize the amount of hydrocarbons being produced. Their only focus is removing the water from the production.
Specialized downhole tools have also been developed, which separate the water from the hydrocarbons downhole. These tools are designed to re-inject the water into some designated formation as the hydrocarbons are produced. While these devices can remove a significant amount of water from the hydrocarbons, their efficiency are usually low. They also suffer from the same drawback of the surface separation devices in that they do nothing to increase or maximize the amount of hydrocarbons being produced.
A solution is therefore desired that not only improves the efficiency and economics of enhanced oil recovery through injection, but that also reduces the amount of injection fluid that infiltrates the hydrocarbon production of an existing well.